For reasoning why a transition to autotools is desired, see the commit
message of the previous patch in this series.
"make install" copies the "build", "common" dirs, as well as the "*.xsl"
files to $(prefix)/share/osmo-gsm-manuals. Prefix is typically
/usr/local. Also a pkg-config file gets installed, so the path of the
shared files can be looked up by autoconf scripts of the project
repositories. The check-depends script is installed to
$(prefix)/bin/osmo-gsm-manuals-check-depends and will be used by
project specific autoconf scripts, too.
All existing make targets ("make", "make check", "make upload") are
still working, users only need to run "autoreconf -fi" and
Makefile.am uses custom install-data-hook and uninstall-local targets,
so we don't need to specify each file of the relevant subdirs in a
_DATA variable (no extra maintenance effort).
(moving manuals to project repositories 5/19)